Various metals and metal alloys absorb hydrogen which can be desorbed once again when the temperature and pressure conditions of absorption are reversed.
Such systems constitute technically valuable possibilities for hydrogen storage, which have the particular advantage of smaller volume relative to storage of hydrogen in compressed gas bottles.
Hydrogen-absorbent materials are known. These include both pure metals such as Mg, Ti, V, and Nb, and alloys, for example, La-, Ti- and Co-alloys, such as Ti-Ni alloys, La-Co alloys, Fe-Ti alloys and La-Ni alloys, as well as Zr-Ni alloys, Mg-Ni alloys, Mg-Cu alloys, rare earth-Ni alloys, rare earth-Co alloys and mixtures of these. Examples include Mg.sub.2 Ni, Ti.sub.2 Ni, LaNi.sub.5, LaCo.sub.5 and FeTi. The temperature and pressure conditions for hydrogen uptake and release can differ considerably, depending on the nature of the hydrogen-storing material. A common feature of all materials, however, is that the stored hydrogen occupies crystal lattice sites in the storage material, i.e., a hydride phase is formed in contrast to adsorptive storage by hydrogen uptake.
An important technical shortcoming of storage systems of this kind is that these substances undergo a constant reduction of grain size as a cyclic consequence of hydrogen uptake and release due to the associated volume change in the metal lattice, and this continues until the volume changes can be balanced by the elasticity of the crystal lattice. At this point the grain size is approximately 1 micron.
Even when originally compact metals or alloys are employed, the latter are frequently converted to fine powder by cyclic hydration-dehydration processes. This circumstance has been used for a long time for the manufacture of metal powders (Zr, Ta) which are otherwise difficult to obtain.
If no precautions are taken, loose powder is obtained due to the familiar pulverization of material during rapid desorption of the hydrogen, known as the "fountain effect," which can cause the bulk material to be carried out of the container in small diameter pipes at high gas output velocity. Even during slow desorption, very fine particles can be drawn into valves and connected systems, causing contamination and leakage of the equipment. Because of the tendency toward plugging the pores, built-in filter plates are viewed only as makeshift devices.
It is known to mechanically harden a hydrogen storage electrode made of titanium hydride in such manner that certain metals, including nickel, are alloyed in it or on it. The metal hydride powder is sintered at 700.degree.-1000.degree. C along with the powdered metal which provides mechanical strength (U.S. Pat. No. 3,669,745). It is also known to envelop by surface contact the hydrogen-storing material with a material which is inactive as regards hydrogen storage, but which is however highly permeable to hydrogen, wherein the hydrogen-storing material is completely covered with a layer of the protective enveloping materials; or a powder made of the hydrogen-storing material and the protective enveloping material is mixed together, pressed into a mold, and subsequently hot pressed to completely envelop the hydrogen-storing material which is then in the form of discrete grains in a coherent matrix made of the protective material (U.S. Pat. No. 3,881,960).
The first proposed solution cited hereinabove does not provide satisfactory mechanical strength to the hydrogen-storing material, and even causes the electrode to corrode more rapidly and turn to powder if unusually high amounts of nickel are not present. Furthermore, such high amounts of nickel have a significant negative effect on the hydrogen-storing properties of the storage material because the surface of the hydrogen-storing material is no longer available for the hydrogen to pass through. Also the high sintering temperatures involved in manufacture constitute an expensive and cumbersome manufacturing step.
The second proposal produces excellent dimensionally stable hydrogen-storing material, but the cost of the TiNi.sub.3 which is conventionally used as the coating material and the care which must be exercised during manufacture in order to reduce a coating for the active hydrogen-storing material which covers the entire surface is likewise costly from the practical standpoint and can be carried out in simple fashion only in the case of nickel-titanium alloys. Moreover, one cannot always be sure that there will be no reaction of the hydrogen-storing material with the added metals or alloys at the suggested sintering temperatures.